Seat belt pretensioners of type initially mentioned are used in motor vehicles and serve the purpose of reducing or eliminating any belt slack which may be present at the onset of an accident by winding the safety belt, and of coupling the occupant as early as possible to the vehicle deceleration. Owing to the early coupling of the occupant, the stress acting on him is reduced by being distributed onto as long a path as possible of forward displacement. So that the available path of forward displacement can also be fully utilized, the belt pretensioner is additionally provided with a force limitation arrangement by which a force-limited withdrawal of belt webbing is made possible. The occupant loading during the force-limited forward displacement is determined by the design of the force limitation arrangement, in which, through a combination of several force limitation elements in a different sequence, different force limitation patterns can also be set.
A disadvantage in such systems, however, is that owing to the successive opposed rotary movements of the belt shaft during the belt pretensioning and the force limitation, the belt webbing withdrawal movement following the retraction movement can be disturbed by the still occurring tensioning force exerted by the belt pretensioner. This disadvantageous force limitation interference can thereby lead to a brief peak at the start of the force limitation process.
It is an object of the invention to provide a belt pretensioner in which the pretensioner drive wheel and the belt shaft are uncoupled as quickly as possible after the pretensioining movement has taken place, to avoid the force limiter interference, in which of course the pretensioning extent or retraction length of webbing is not to be shortened.
The object of the invention is achieved by a belt pretensioner having the features described herein.
According to the invention, it is proposed that a retaining element is provided for coupling the pretensioner drive wheel to the belt shaft, and the retaining element is releasable by the rotation of the belt shaft in the belt webbing withdrawal direction after the pretensioning movement has been completed. Owing to the proposed retaining element, the pretensioner drive wheel and the belt shaft are initially coupled with each other so as to be locked against relative rotation, so that the pretensioning movement is transferred to the belt shaft in a manner which is as free of loss as possible. The releasing of the retaining element then takes place by the onset of the belt webbing withdrawal movement, so that the force limitation in fact can no longer be disturbed by the force exerted on the belt shaft by the pretensioner drive wheel.
So that the uncoupling of pretensioner drive wheel and belt shaft takes place as immediately as possible with the onset of the belt webbing withdrawal movement, it is proposed that the retaining element is able to be moved from the securing position into a position of readiness by the rotary movement of the pretensioner drive wheel. In the position of readiness, the retaining element still couples the pretensioner drive wheel with the belt shaft, but is already in a position from which the releasing of the connection takes place with as little loss of time as possible.
The releasing is preferably then brought about by the onset of the belt webbing withdrawal movement, in which here the advantage can be utilized that the belt webbing withdrawal movement is directed in a direction opposite to the pretensioning movement, so that a releasing of the retaining element in error can still be ruled out during the pretensioning movement.
It is further proposed that the pretensioner drive wheel and the belt shaft are coupled with each other via a ramp contour, and the releasing movement of the retaining element is able to be activated by the carrying out of an axial movement of the pretensioner drive wheel which is forced by the ramp contour. Here, the proposed ramp contour can be utilized both to move the retaining element out of the securing position into the position of readiness, and also for the actual releasing from the position of readiness into the releasing position. The ramp contour can also be designed here so that the deliberately forced axial movement only takes place until the position of readiness is reached, and then a force transmission is possible in the peripheral direction, in order to transfer the actual pretensioning movement.
It is further proposed that a clamping surface is provided on the belt pretensioner housing, on which surface the retaining element is able to be fixed by carrying out the belt webbing withdrawal movement, and the releasing movement of the retaining element from the belt shaft and/or the pretensioner drive wheel is brought about by the fixing.
It is further proposed that the retaining element is formed by a spring plate which is keyed with the belt shaft. The proposed spring plate can be produced as a favourably priced stamped part. In addition, with a corresponding selection of the spring steel, the spring plate fulfils the mechanical material requirements with regard to the force assimilation during the pretensioning process and also during the changeover from the retraction movement into the belt webbing withdrawal movement. In addition, the choice of the spring steel makes it possible that the retaining element is prestressed elastically in the position of readiness by the pretensioner drive wheel, so that the retaining element automatically moves into the releasing position after the rotary movement has been carried out at the start of the force limitation.
The spring plate can have a bore with an inner toothing, and can be slidable onto a shoulder of the belt shaft which is provided with an outer toothing, in which the pushed-on spring plate is able to be fixed by rotating and keying on the belt shaft. The spring plate is thereby deliberately keyed by a directed rotary movement which, in the opposite direction, is used by the onset of the belt webbing withdrawal movement of the belt shaft for releasing the spring plate.
So that the onset of the belt webbing withdrawal movement is converted directly into a releasing movement of the spring plate, a stop is provided on the pretensioner drive wheel, against which stop the spring plate lies. During the mounting of the spring plate, the stop serves as a limit to the rotary movement which is necessary for the keying. As the pretensioner drive wheel, during the conversion from the belt retraction movement into the belt withdrawal movement of the belt shaft is acted upon with the pretensioning force still present in the retraction direction, the stop can be used in addition as a quasi fixed stop surface for the spring plate which is connected with the belt shaft. The spring plate is thereby additionally fixed with respect to the pretensioner drive wheel.
It is further proposed that the pretensioner drive wheel has at its end face facing away from the belt shaft a taper against which the spring plate lies and the spring plate carries out a swivelling movement by carrying out an axial movement of the pretensioner drive wheel and thereby fixes itself against the belt pretensioner housing. The proposed taper makes possible a linear or punctiform abutment of the spring plate and, with an axial movement of the pretensioner drive wheel, produces the required swivelling movement of the spring plate for the application of the clamping force.
The swivelling movement is further simplified by the spring plate having spring arms spaced apart on its outer periphery, and by lying with the spring arms against the taper. Owing to the spaced spring arms, firstly the spring rigidity is deliberately reduced, so that the swivelling movement is carried out reliably and, in addition, the spring arms can carry out a fan movement, directed towards each other, during the swivelling movement.
A further preferred embodiment of the invention consists in that the spring arms project in the axial direction from the spring plate and, whilst the axial movement of the pretensioner drive wheel is being carried out, carry out an inwardly directed swivelling movement, whereby the spring arms draw together and the necessary clamping force can be produced in a structurally simple manner.
A further preferred embodiment of the invention consists in that the spring arms in cross-section form at least partially an obliquely directed ratchet contour, and the clamping surface of the belt pretensioner housing has a counter contour which permits a relative rotation of the spring plate to the clamping surface in one direction and with a relative rotation of the spring plate to the clamping surface in the other direction secures the spring plate. The spring element can thereby already be brought during the pretensioning into the position in which it is fixed with the reversal of the direction of rotation of the belt shaft without a further adjustment movement with respect to the belt tensioner housing.
In addition, it is proposed that the spring arms of the spring plate have a mechanical coding defining the installation position, so that an incorrect installation is prevented and an automated production process is made possible.
The coupling of the pretensioner drive wheel to the belt shaft can be further improved by a blocking element being provided between the belt shaft and the pretensioner drive wheel, and by the blocking element being secured in the blocked position by the retaining element.
The retaining element can be formed for example by a band, in which a first end of the band secures the blocking element in the blocked position, and the second end is able to be fixed with respect to the belt pretensioner housing by carrying out the belt withdrawal movement.
A simple type of fixing of the band consists in that the second end is able to be fixed by winding around a clamping surface arranged on the belt pretensioner housing. The rotary movement carried out by the belt shaft is thereby also used as a winding-around movement of the band about the fixed clamping surface which is necessary for fixing the band.